I. Field of the Invention
This invention generally relates to a device and method to prevent the counterfeiting of paper money, stock certificates, or similar documents. More specifically, it relates to an image recognition device which is capable of recognizing a specific input image even though the input image has been enlarged or reduced in size.
II. Description of the Related Art
In recent years, the capabilities of full-color copy machines have become so sophisticated that it is often impossible to distinguish a copy from the original image with the naked eye. At the same time, such copying machines have become readily available. Thus, there is an increased danger that copiers will be misused to counterfeit documents, which may not legally be reproduced, such as banknotes and stock certificates. Various devices have been developed to prevent such counterfeiting.
Some of these devices pre-record a specific image of a banknote or other document, which may not be legally reproduced. Then, these devices determine whether the pre-recorded specific image exists in an input image which someone is attempting to copy. The method used to determine whether the image is recognized (i.e., pattern matching) entails evaluating the input image's distance from the template, or the relative correlation between a target pattern (the template) and the input image the device is attempting to recognize (the object of recognition).
If the device does recognize the target pattern in the input image, it employs any of various means to prevent copying, such as covering the paper entirely with black ink or halting the process.
Size is an essential element used in the technique described above. The size evaluation consists of determining if the input image and target pattern are exactly the same size. The input image and the target pattern must be the same size for the device to prevent copying. If the shapes in the two images are the same (i.e., the figures are similar) but their dimensions are different, the device will not recognize the input image as identical to the target pattern. As a result, if someone were to use the copier's zoom function to reduce or enlarge the input image to be copied, the size of the input image data, which the image recognition device received from the copier, would not match the target pattern stored in the device, and the device would conclude that the input image was not one for which copying should be prohibited.
A digital color copier normally reads the image loaded on the glass by moving an image sensor with one line of CCD (in the scanning direction) at a given speed. If the input image has been reduced or enlarged, the speed of the image sensor can be increased or decreased respectively. This is necessary because the quantity of data in the feed direction, which is received per unit of time, will differ with magnification.
It is understood that humans recognize shapes by a complex process. However, human are not as skilled in evaluating size. In fact, recognizing (or discriminating) small differences in size is difficult for most people. An item which is successfully copied because the image recognition device does not recognize the input image as the same size as the reference image may nevertheless be indistinguishable to the human eye.
If the magnification (either enlargement or reduction) is ten percent or more, anyone can easily recognize a copy by comparing it directly with the original bill or document and seeing that it is the wrong size; but seeing only the copy makes it much more difficult. Furthermore, if it is a bill which a person seldom sees, foreign currency for example, the task is even harder.
To address this problem, the image processing device disclosed in Japanese Patent Publication 6-237379 was developed. During preprocessing, this invention extracts a specified portion (which varies with the magnification) of the input image data based on the magnification it receives from the copier. This process makes the resolution of the extracted image data fixed at any magnification or zoom level. In this way the same processing can be executed in every subsequent unit without regard to the magnification, so that the likelihood of recognition will not vary with magnification.
The prior art device described above is effective for machines such as copiers which can transmit magnification or zoom data to it, but it cannot be used for machines which are unable to provide accurate magnification data. For example, an image processing system which consists of a scanner, a personal computer and a printer, inputs processes and reproduces the image by itself. This makes it difficult to obtain the sort of reliable index of magnification which a copier can readily provide. Thus, the recognition processing employing magnification disclosed in the Japanese Patent Publication 6-237379 cannot be applied to this sort of image processing system.
The rapid development of electronic technology has improved the resolution of all the devices discussed above and enabled accurate full color printing. For this reason we must have a reliable way to halt the reproduction of images containing specified target patterns.